.308 Subsonic out of HK-91 A2

[arrowshot]

I just discovered my HK 91A2 barrel is reportedly a 1 to 12 twist (pre-ban HK out of Virginia purchased in 1984) and now I am really concerned about the subsonic loads I have been using with my rifle suppressed with my 76.2SD. I got the following results a few months ago.

HK-91, .308, Trail Boss Powder, Sierra 168gr hollow point boat tail
11.0gr (without suppressor) - 1027, 1114, 1106
11.0gr with suppressor - 1114, 1118, 1124
10.8gr - (without suppressor) - 1116, 1096
10.8gr with suppressor - 1131, 1101
168gr boat tails all tumbled at 50 yards.

After reading here for a few years or so now I know a 1 to 12 twist is not really the best for subsonic rounds. Would appreciate some opinions/advice regarding the HK as a platform for subsonics and for my own safety and to avoid damaging my suppressor. I have already searched the forum and found all the postings on HKs but still looking for more advice. Would a lighter bullet work better with less expectation of tumbling? How about a heavier bullet( 200gr)?

[johndoe3]

Use the cleaning rod method with a tight patch to actually measure the twist the you have.

1in12 twist works fine for subsonics, but you have to stay lighter on the bullets. 168gr should be good, and maybe the 180gr flatbase roundnose bullets on the high end. However, if you use 180gr bullets, shoot without a suppressor and make sure none keyhole, before shooting through a suppressor.

A bullet that may be slightly unstable in the middle of winter may be fine in the middle of summer (like 180gr for you). Stability is temperature dependent (density altitude of the air).

[silencertalk]

I would change to a 150 grain round-nose 30/30 bullet.

[arrowshot]

I would change to a 150 grain round-nose 30/30 bullet.

Thanks. Will order a hundred and see how they work out.

[arrowshot]

Robert,
Here were my results:
2010-07-02
HK-91, .308, Trail Boss Powder, Sierra 150gr round nose 30-30 bullets
10.5gr – 1170, 1045, 1023
10.0gr – 1084, 1034, 1034
9.7gr – 951, 1010, 1009
9.3gr – 804, 953, 922
• Failed to cycle on all rounds of course, thats normal. Still somewhat consistent on FPS numbers though. No tumbling bullets through and very good accuracy

A big question of concern. When I load the Sierras up to cannelure I end up with an OAL of 2.535 (maximum for .308 is 2.80). This is considerably shorter than the OAL I get when loading my hunting or other bullets. I know that bullets should be seated just off the lands but could this result in any problems? As I noted, I had very good accuracy and no tumbles.

[silencertalk]

I know that bullets should be seated just off the lands but could this result in any problems? As I noted, I had very good accuracy and no tumbles.

It is fine to have a lot of bullet jump with beefy bullets like that. Just not great for thin-jacket varmint and benchrest bullets.

You should use 9.9 grains.

[arrowshot]

I know that bullets should be seated just off the lands but could this result in any problems? As I noted, I had very good accuracy and no tumbles.

It is fine to have a lot of bullet jump with beefy bullets like that. Just not great for thin-jacket varmint and benchrest bullets.

You should use 9.9 grains.

I agree and will be loading up a few more. Still kind of concerned about the FPS variation though. Do you think the Trailboss slides around inside the case resulting in the variation?

[Bowen1911]

Have you tried magnum primers? I use CCI LR magnums in my 7.;62x54R subsonic loads and I have been getting really consistent loads if I weigh each one, and sort my cast bullets by weight.

[silencertalk]

No, I don't think the variation is cased by the powder sliding. I have not yet figured out if Trailboss is better than using a small amount of fast powder.

Both have their own set of issues.

You can try it with and without magnum primers and see if the standard deviation goes up or down. You will need to fire at least 10 and ideally 30 or more rounds.

[arrowshot]

Have you tried magnum primers? I use CCI LR magnums in my 7.;62x54R subsonic loads and I have been getting really consistent loads if I weigh each one, and sort my cast bullets by weight.

Not casting bullets since I don't want to clog up my 7.62 SD. Do that for my black powder though.

[arrowshot]

No, I don't think the variation is cased by the powder sliding. I have not yet figured out if Trailboss is better than using a small amount of fast powder.

Both have their own set of issues.

You can try it with and without magnum primers and see if the standard deviation goes up or down. You will need to fire at least 10 and ideally 30 or more rounds.

Might give that a try also and see if it has a different result. Did want to shoot more than 4 each but all I had time to put together this time. Really happy to get some bullets that didn't tumble though.